I have an unexplained bruise on my knee/thigh. Could this be a sign of a torn muscle?

Maybe. Should be evaluated by your medical dr. To r/o bleeding diatheses first.
Yes. A muscle tear can cause bleeding and that would manifest as a bruise that could be seen below the area of pain. (blood drains downward due to gravity).

Related Questions

I had calf muscle tear with horrible bruising and pain. Am I more prone to that now in that leg? I have 2 knee replacements.

Calf Tear. Sounds like you had a grade iii muscle tear. This leads to weakness that much be regained by gradual restrengthening. It also leads to swelling and scar tissue that needs to be worked out. Make sure you spend time with a manual physical therapist who will work out all of the scar adhesions and regain the strength and flexibility. They are great at that. Dr rich blake. Read more...
Calf Tear. Unlikely. After it is healed up, concentrate on strengthening up the calf muscle. Your doctor or a personal trainer can teach you what exercises would be most appropriate. Read more...