Long story short: my gf is Russian and recently visited Moscow for the first time in 2 years (for 15 days).

Yesterday, she received jewelry and a love note from one of her "friend" in Moscow. Her story: she hasn't seen this guy in ten years, but chatted remotely occasionally. She says he went crazy when she refused to see him during her visit.

But this story doesn't make sense to me. Why would a guy who she hasn't seen for ten years (and only knew him briefly before that) would suddenly mail her jewelry. I think the most likely explanation is that there was a fling in Moscow that she is hiding.

Any thoughts/suggestions? I want to trust her, but the logic isn't working for me.

And if I let it fly, how do I do it in a way that is alpha/demonstrating that I am not going to put up with any of this shit in the future?

and to add: What I think I'm going to do is let it slide and say something like:

"I'm going to trust you - but I don't want to see this shit again. I want you to make it very clear to every guy you talk to that you are mine. There's no reason that a guy should think he has a chance with you"

There's of course different options, this is what I would do: If you are a calm guy who doesn't let his emotions get the best of him, tell her that her story doesn't make sense to you and ask her why this guy would send her jewelry when he hasn't seen her in 10 years. Then tell her if there is more, now is the time to come clean.
If she still sticks to her story, let her know you're gonna trust her with this one. But in general you do not accept cheating/lying, and should she ever cheat/lie to you that would be the end of the relationship.