one reason a horse might not show lameness in a new situation is that it's so stimulating that they move out with energy, from their fear/anxiety/excitement about the new surroundings. They will not notice the pain, I guess.
I've ridden a horse that seems slightly lame in an off/on manner, where I am not sure where, or if, they ARE lame. I continue to ride, experimenting with this and that to see if it helps me discern what/where the problem is. If they are actually limping from the get go, or every single time we take up a trot, then it's time to get off.