Note: This sign can be used to mean "inside" by doing the movement twice but using a smaller motion the second time.
To sign outside you can just use an exaggerated version of the sign "OUT" or you can visit this page for more variations: OUTSIDE
At your convenience. Thanks.
That sign could be "ENTER."
Especially if the left hand has a bit of a curve.
When you say, "behind it" if you mean right hand swoops under the left the thumb side of the palm down left flat hand and comes out on the pinkie side of the left hand as if a small animal entering its burrow, then I think that is definitely your mystery sign.
Here's an example:
American Sign Language University ™ ASL resources by Lifeprint.com © Dr. William Vicars