It is possible to tell.
I had a teacher in college who had a story about how she made some silhouettes for a zoo and for one of them she used a photo of an eagle she found on Google or wherever.
Cue the photographer of that eagle finding out.
Apparently, the photo of the eagle was at a specific angle that was difficult to take. So it wouldn't be impossible if someone knows the program well enough to figure it out.
But, yeah, CSP makes it incredibly easy to draw a background that looks decent relatively quickly so just do that.