Why did Virgil hide in the The Glowing Sea?
Solution 1:
Virgil believed, correctly, that Kellog would be sent after him.
Kellog is not immune to radiation.
Solution 2:
Based on institute main questline, synths can only be sent to areas without an em(electromagnetic) field in place. In physics and chem, radiation is a rich field of charged particles and creates own em field.
Solution 3:
Virgil hid in the glowing sea, hoping that radiation would cover up his tracks.