One way is to move your mattress over (a bit), stick it in the side, and cover it with your sheets.
If you have a gap between your mattress and the headboard, you can stick it in there.
You can also hide something behind books...
...or stick it in with the books
Lean your item on the window and pull the shades down (this is only a TEMPORARY hiding spot, don't use this for long time things), and check for shadow.
Hide it behind a bookshelf or nightstand, try not to leave too much of a gap between the item and the stand.
And finally, you can just go with the original ones: under your pillow, bed, covers, etc. Thanks for reading, and I hope you found this helpful!