Trespassing is trespassing, it doesn't matter if they were chasing a dog or not. For all you now they were using it as an excuse to scope out your place so they can come back and relieve you of some of your belongings. I wouldn't tolerate it ether.
Exactly, you never know what peoples intentions really are, despite what they might say. If I intend to go onto someones land for any reason, I ask them. It's their land and I always respect that fact and expect the same in return.
Many years ago, the road to our home was flooded and the only way in or out was by using my ATV to cross a large field a neighbor uses to grow hay. Fortunately, he had already harvested it for the season so there was no risk of causing any damage so I proceeded to cross the field and go to his home to ask permission. In this case, I had no choice but to cross the field to ask permission, which he understood and granted me permission, but only while the road was flooded.
Respecting boundaries is just the neighborly thing to do. Period.