To find SIL's behaviour a bit odd.

[GymOClock22]

I'll start by saying I have nothing against SIL. We aren't really friends but will have a chat at family gatherings. I'm perfectly willing to hear I am being PFB about this.
I'm currently pregnant with mine and DH's first baby. We had scans a few weeks ago and got some prints for both sets of our parents.
We posted the scan on FB to let friends know (I know this isn't the most popular thing on MN).
Today I went round SILs. I very rarely go to her house but she had packaged up some baby things for me. Which I am grateful for. However when I went into her utility room with her I noticed that she had my scan picture pinned to her noticeboard. Isn't that a bit strange? I was taken aback at the time so didn't say anything. Would this make you uncomfortable?

Why do you assume it's your sil and not your actual brother who has put the picture up?! If you don't have a close relationship I very much doubt she'd do it. Are you sure it's even your scan? If it is, maybe your pils printed it for them so they just stuck it up as they didn't know what else to do with it. If it's stuck in the utility it's not exactly pride of place where people are going to usually see it, it could even just be a helpful reminder when they see it to check in with you to see how you're getting on..

Agree with others, she's probably excited about becoming an auntie. Nothing sinister.