strange bluetooth pairing request

You obviously need it then... I would try going into Bluetooth preferences, and unchecking "discoverable". That may help prevent the annoying requests and still allow your trackpad to connect with your iMac. I just turned off my discoverability and my Bluetooth keyboard works fine, so that may help you! Good luck getting rid of annoyances!

Yup, just a nosy person, or malicious. If you aren't using Bluetooth, I would just turn it off until you need it ( unless you have a b.t. Keyboard or something). You can actually search for devices from your iMac and see what come up. That's how someone found you. The b.t signal doesn't stop at your walls! Unless, of course, your walls have anti-bluetooth penetrating paint! Lol. Just be cautious, and keep an eye on your stuff! Like I said, I would turn it off if you don't absolutely need to have it on.