From a hill in this West Germany village, U.S. military border patrols frequently turn their field glasses on the town of Vacha just across the East German border.

Vacha is a quiet place, and the Soviets and East Germans clearly know it is easily observable from the West.

Yet a few months ago, the town figured in the tantalizing cat-and-mouse game that goes on secretly between the military forces of East and West.

There is not much noteworthy about Vacha, except perhaps for its railroad yard. Last January, according to officers of the U.S. 11th Cavalry who patrol the border, several empty flatcars rolled into those yards.

Two nights later, at a U.S. border observation post several miles south of here, soldiers recorded more than 100 "sightings" of heavy vehicles across the border in East Germany. The "sightings" were made on small radar sets and by devices that sense thermal, or heat, radiation.

This had not happened before in the memory of the GIs on duty that night, some of them three-year veterans on the border, and communications lines to headquarters in the rear got a lot of use.

During the day, the fields in front of the observation post were clear, but at night it was also clear from electronic and heat signals that many tanks were operating just across the border.

The following day, 42 Soviet-built 62 tanks were visually spotted moving along an East German road.

"Obviously they dumped their tanks somewhere before reaching Vacha," said U.S. Army Maj. Henry Lowe. This would explain the empty flflatcars coming into the rail yards. "So they snuck the tanks in on us," he said.

At the same time, a number of officers from the Soviet Military Liaison Mission in the west reportedly paid calls at unusual hours on various U.S. military headquaters areas in the rear.

The general assessment here is that the Soviets were trying to see if there was any noticeable allied reaction to the not-so-secret tank maneuvers just across the border.

"We reacted," said one officer, "but not in a way the Soviets would notice."

The incident was both rare and potentially important, in part because this strip of border leads through the so-called "Fulda Gap," a traditional attack route in Central Europe that leads directly to Frankfurt and the Rhine splitting West Germany in half.

Without referring to this incident, regimental commander Col. Crosbie Saint, said, "The reason we are spending so much of our time now working on the border is so we know what is unusual," perhaps a reference to the flatcars.

For intelligence officers, the incident raises many questions. Could the Soviets move much larger numbers of vehicles very close to the border without being detected? Were they testing the Fulda Gap or suggesting such a route as a decoy? Were they only watching Western reaction times or were they also trying to plant false information about where these particular units may have come from in order to confuse Western defenses?