After reading Indiana's code, it would appear entering someone's property to leave a note would not be trespassing unless:

1. They crossed a fence
2. There are clearly marked "No trespassing" signs ("iffy call")
3. They were verbally told not to enter the property, before-hand

http://codes.lp.findlaw.com/incode/35/43/2/35-43-2-2

I'm not a lawyer, but it seems pretty clear.

Some states have "with unlawful intent" in their code as well.