Did you put up no trespassing signs?

NO TRESSPASSING : All violators will be put to the sword

If not put those up first then come back and ask

Have a dilpomat within distance to a Numidian unit/city. Go kick the armies butt then ask for a ceasefire. If they don't accept the ceasefire they were planning on using that army in the near future more than likely. If they do accept they were scouting with there army and nothing else.

As far as getting bad diplomatic points. I do'nt think you will. At least you should'nt. You only get bad points when you cross borders or have an army in neutral territory for more than 1 turn from my observations.