There are typically three forms of death.
The first is charge leakage (which also eventually affects Flash), where the 'programmed' charge one the floating mosfet gates leaks away over the years. This causes 0 bits to slowly (going through intermittent) transition to 1s. As long as you have an image of the flash code, it is normally possible to re-write the same UVeprom and achieve pretty much the original life again.
The second type is 'Sunburn' This used to be a common problem in labs where people set the UV lamp timer too long - particularly in labs with shared erasers where thoughtless people would put their eproms in and reset the timer without taking other peoples' out. This results in bits stuck at 1, and impossible to program to 0 again.
The third type is bits stuck at 0 after erasing particularly after several cycles of use. Sometimes curable by extending the erase time on well-used eproms for a few more cycles, until you eventually lose them to the effects of 'sunburn.
Obviously types two and three are destined for the bin. Type one is reversible (as long as you've remembered to capture an image in good time).
P.S. It's important that the window is covered, preferably with a UV blocking Aluminium label for maximum life (I'm talking decades).
P.P.S In terms of your specific question. Leaving an eprom in a sunny window, especially in a sunny place like Zambia, might cause erasure over a week or two, however if the sun is shining through glass, then that would block the needed UV-C wavelength. I suppose it is possible that operation in strong light could cause excessive on-die leakage currents, but probably not to the extent of self-destruct. A few people actually had some success using UV-eproms as primitive imaging chips in the past. If the eprom had any sort of label on the window, this is pretty unlikely.