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That really depends on what you mean by "separated." There are varying degrees of separation. If you aren't completely done with your military service (for example, if you are going on any terminal leave), you can have that leave revoked, be placed on legal hold from separation and have legal action taken against you.
So, a lot here depends on what status you'll actually be in, when this is discovered and how much the commander actually cares about it.
Even if you are no longer in military status, they could technically have you charged under federal law with possession/use because if you were on a military installation, you are also subject to federal criminal laws. I've not seen this done, but it could be done.
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