On the inside of the non drive side seatstay on a 6-ish year old Trek 7500 hybrid are 2 bottle cage sized bolts quite close together. There’s nothing on the opposite stay.
Like this:

Untitled by PeterPoddy, on Flickr

What are they for then? Any ideas? 🙂

Stabiliser for an unbalanced Aussie?

Dynamo

Dynamo

Really? 🙂

a very small water bottle?

Stop pissing about on STW and get back to work!

I’m thinking something lke this…

..though that only has one mounting bolt.

rocket launcher?

Non-UCI-legal fareing?

Caravan?

Falconry Perch?

Can’t tell how high up the stay they are. If they are near the hub could they be for a coaster brake arm? Does look a bit high for that and not likely on a geared hybrid, more likely on a cruiser or town bike.

is it to hang pictures from?

Dynamo. There’s a bracket that bolts to that, that takes the standard single mounting bolt for a dynamo.