The explosion aboard TWA Flight 840 on Wednesday has revived worries about how, despite upgraded security at several Mediterranean area airports, the antiterrorist net easily was pierced again.

Some Egyptian officials today still were rejecting the idea that the bomb might have gone aboard TWA's 727 in Cairo, as is widely believed. "This is a silly story that calls for no comment on our part," said a government spokesman quoted widely in the Egyptian press.

In Washington, the Egyptian Embassy said it was impossible that the explosive was smuggled aboard in Cairo. But a European airline manager in Cairo said that, privately, "the Egyptians are actually very upset."

Questions arose, too, about how the bomb could have gone undetected at Rome's Leonardo da Vinci Airport during the two hours when the plane was being cleaned, spot-checked and refueled there before it took off for the trip to Athens.

While security at airports in Rome, Athens and Cairo has been stepped up following terrorist incidents in recent months, gaps remain.

"Like anywhere else in the world," said one airline regional manager in Cairo today, "your security checks are as good as the people you employ."

An airline manager who has received extensive briefings on security matters in Cairo noted that high-powered plastic explosives do not normally show up on a "walk-through" metal detector. The detonator can be small enough to hide in the cap of a pen. It is possible to mold plastic explosives into the soles of shoes.

"You can walk on it, play football on it, and nothing happens unless you put a detonator in it," said this official.

Italian airport security was upgraded early this year following a terrorist attack Dec. 27 on the TWA and El Al check-in counters at da Vinci Airport that left 15 persons dead, including five Americans.

As a result of that attack, and continuing intelligence about the possibility of further attacks, police patrols outside and inside da Vinci have been increased greatly.

More guard dogs (able to smell explosives as well as drugs) have been assigned to the patrols, check-in procedures are supposed to have been streamlined, and X-rays of check-in baggage as well as carry-ons have become routine, especially on flights of U.S. airlines or those heading for Israel, Egypt and other Middle East points.

But many travelers have complained that Italian checks of carry-on luggage appear inefficient. Persons carrying complex electronics, such as portable computers and tape recorders, or even gallon cans of virgin olive oil, are almost never double-checked.

Airport security personnel, who declined to be identified, also said there are no rigid checks on air freight from points abroad or on transit baggage of passengers changing flights in Rome.

In Cairo, several airline employes, asked today about possible changes in security, said they thought that short of imposing Israeli-style searches, there was little room for major improvement.

The airlines' calculations are based on a balance of security and convenience for the passengers, according to airline employes. Safety is a "very, very high priority, but then comes the commercial objective," said one manager.

In Israel and on El Al flights leaving Cairo, passengers are liable to have everything they carry or check through searched by Israeli security officers. The main screening appears to be based on a thorough perusal of the passenger's passport and a sometimes lengthy interrogation: When were the bags packed? Who did it? Were they left anywhere where they might be opened? Did anyone give you anything to carry?

Among foreign nations, only Israel is permitted to carry out physical examinations of passengers' luggage and persons in Cairo.

There are plainclothes security people in some of the shops on the main concourse. According to one senior airline employe, these people spend "more time watching people than selling papers."

In addition to guards patrolling the concourse with automatic weapons, there is a sniper position on the mezzanine above it where a soldier with a rifle and telescopic sight was visible today.

Airlines and travel agents advise passengers leaving Cairo to be at the airport two hours ahead of departure time. Cairo security officials cite five separate checkpoints, although most travelers might consider some of them fairly theoretical.

The first is at the entrance to the airport, where a guard at the door may spot-check luggage, but this is rarely done. The second is at the TWA counter, where bags are X-rayed and sealed with tape. There is a customs check behind the TWA counter, sometimes limited to cursory questions. Hand luggage is X-rayed at the entrance to the departure hall. Then a final, and usually fairly thorough check of hand baggage is made at the gate. This includes a pat-down body search, with one line for men and another for women.

"We've been pushing about the quality of the women doing the search," said the manager of a European airline. "The screening of women here is a weakness -- and at other airports. A lady with a baby. How sweet. Zap!"

In Washington, the Egyptian Embassy said today that the woman now suspected of planting the bomb was carefully searched before boarding. "Because of the time required for the body search and the luggage search, she was taken to the plane by a TWA security officer at Cairo," said Abullaah Hafez, an embassy spokesman.

Some airline employes in Cairo speculated that because of the rush of the woman's last-minute arrival at the airport, the female agent at the gate might have given her a cursory inspection at best.

Hafez said, however, that the woman was given a thorough body search and her luggage was carefully screened "because she was suspected" of something.