If a pawn has just moved two squares ahead, one records the square right behind it in the part of FEN for recording en passant possibility, regardless of whether there is a pawn of the opposite color ready to make the capture.
For example, the FEN for the position after 1. e4 is:
rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1
Why is this done? It sounds superfluous, since an en passant capture can be made only on the very next move of the opponent's, so recording it as such when there is no pawn in position to make the capture seems unnecessary.
Is there some historical or technical reason for this?