I remember reading that the IR sensors on the Eurofighter Typhoon can pick
up icebergs against the cold of the sea!

However I also remember reading that the IR sensors on the MiG29 has a very
limited range, just a very few miles maybe less than 10.

Based on these two comments a Stealth fighter can be picked up but you must
be extremely close to do so. IR sensors by themselves don't seem to be the
best way to find these aircraft.

MikeT