You go into the bathroom and take off your trousers.
You hold the stain close to your face and look at it.
It is definitely ketchup.
You run the stain under the tap for a bit and rub at it with your finger.
It goes away, mostly.
There is just a small mark.
No one would notice unless they were looking really hard.
Well done.

do you:

put your trousers back on
leave them off and walk around in your boxer shorts for a bit