You concern is justifiable, but it can be anything from masturbation or infidelity. If infidelity, you do need to be concern, because you don't know if he has had unprotected sex and come home with potential std's or HIV. Communication and trust are key to all relationships and I suggest that you find the right time to firmly "discuss" your concern and an explanation as embarrasing as it might be for him and for him to come home with stained cloth's is questionable. He probably didn't have spare cloth to change, so, yes, your concerns are justifiable and expect him to give you an explanation, before deciding on the next step. Good Luck, Judy
Dear Nick661
Semen strains can be found because your BF had a nocturnal ejaculation during his sleep. This happens to men who have not had sex or masturbated for some time.
And the smell of semen when he was aroused may had been there because he got a little overexcited and a small amount of semen had been ejected.
The other explanation could be cheating, so you still need to talk to your BF about it but try to do it gently so that he will not get angry...
Best wishes John