Do men on occasion release semen without realizing it (say during the day) and so they may see a small stain on their underwear? how normal is this? is is harmful to others if they sat somewhere or used a toilet seat that someone else may use after?

Leaking semen doesn't happen, but if you are discharging mucus or cloudy fluid, you likely have an STI like gonorrhea, chlamydia, or nongonoccal urethritis. Or maybe it's just urine. Or your just over-worried, as in your other questions about prostate fluid, dusty rooms, and dog treats. Your germophobia and OCD are showing. You probably need professional counseling, if not already in such care.

Small amt. of discharge you describe is not semen (ejaculate) and can occur normally or in sexually excitation. Toilet seats should be cleaned or paper cover applied when using public toilets .