There is something suspicious lurking among the tiny islands that dot Sweden’s coastal capital city of Stockholm.

On Friday and Saturday, Sweden mobilized ships, helicopters and more than 200 troops following reports of “foreign underwater activity” less than 31 miles off of the coast. The operation, reminiscent of an era when Sweden routinely patrolled its coastal waters for incursions by Russian submarines, prompted Cold War comparisons to the 1990 thriller “The Hunt for Red October” and the 1981 “whiskey on the rocks” incident, during which a Soviet spy submarine ran aground near a Swedish naval base.

On Sunday, the Swedish military reported three credible sightings of what they describe as “foreign undersea activity” and published a picture of a partially submerged object.

Also on Sunday, a Russian-owned oil tanker, the Concord, which had been in Swedish waters for several days, started sailing toward Russia before suddenly turning around and heading back to Sweden, the Associated Press reported.

So far, Swedish officials have declined to name names. Speaking to reporters this weekend, Rear Adm. Anders Grenstad would only say that the region is “of interest to a foreign power.” Stockholm is a day trip by sea from the Baltic states of Estonia, Latvia and Lithuania, three former Soviet republics.

The Svenska Dagbladet newspaper reported (link via the Guardian) a Russian mini-submarine ran into trouble in Swedish waters, citing a radio conversation in Russian on a channel used for emergencies intercepted by intelligence officials on Thursday. But the military said it received no such reports, according to the AP.

Another signal was intercepted Friday night, Reuters reported. The message was in code, but Swedish intelligence officials said the call could be traced to Kalingrad, the Russian port city home to its Baltic fleet.

The AP reported Moscow’s defense ministry said its submarines and ships are “fulfilling their tasks in the world’s oceans” according to plan. “There have been no emergencies or accidents with Russian military vessels,” an unnamed spokesman at the ministry was quoted by Russian news agencies as saying.

Russia’s Nordic and Baltic neighbors have been on guard since the former superpower invaded Ukraine earlier this year. In September, two Russian Su-24 attack jets reportedly violated Swedish airspace in what Foreign Minister Carl Bildt called “the most serious aerial incursion by the Russians” in almost a decade.

Last week, Finland accused Russia of interfering with environmental research vessels twice in recent months. They were twice ordered to leave international waters near Sweden by a Russian naval vessel and noticed a Russian submarine nearby. Finland also accused Russia of violating its airspace three times in one week in August.

“What’s been happening in the Baltic Sea, including airspace incursions, shows that we have a new, changed situation,” Sweden’s recently appointed defense minister, Peter Hultqvist, told Svenska Dagbladet. Hultqvist, a Social Democrat said his government would be more transparent about military incursions in the Baltic than its center-right predecessor.