I'm no hunter and I don't necessarily trust a Google search, but Ansitzsack seems to be the common version, while Ansitzschlafsack gets very few hits. Here's Ansitzsack:
https://www.frankonia.com/jagd/ausruestung/jagdbedarf/koerpe...Looks the same to me too.
And a simple "sleeping bag" is still wrong, IMO. You may get to hide/blind, but to "mummy shape." This is from a UK company:
http://www.military1st.co.uk/sb122-highlander-cadet-350-camp...Have a look at their selection:
http://www.military1st.co.uk/camping-and-survival-sleeping-g...Nothing comparable in there, I'd say.
However, herbalchemist's mentioning of babies got me an idea. That's kind of similar:
https://www.planetcamping.co.uk/selkbag.htmlIt's a "sleep(ing) suit," not just for children. Similarly (first one):
https://www.pinterest.co.uk/pin/812759063973535857May not work, though. Else: "wearable" or "with sleeves" (see below). She's right that you need
something.
Best