If there's like a yellow patch, touch it, smell it before
confirming.
If it smells like his usual cum, then he either wanked it, then
wore his undies back without cleaning his didi, or got sexually
aroused by some visuals while outside, or he's cheating on you.
If it doesn't smell like his cum, IT'S PEE or his undies isn't
washed properly (got mouldy).