Quick background, broke up with ex girlfriend about 10 months ago.. NC for 2.5 months, she in long distance thing with this new guy, she last contacted me by email about month and half ago to which I gave no reply. Today coming into work I run into a mutal work friend (ex has same work type, we could theoretically cross paths) I go to talk to this girl and notice ex standing like 10ft away with group of other work people, which I also know. Ex's back to me. I just quickly chatted to this other girl and then headed off in other direction, I noticed that couple work people she was with did catch sight of me. But I just turned and walked, no word to ex. Part of me thinks I should have said hello, to be honest I almost got this panicked feeling when saw her.. I feel bit immature like that was cowardly to do. Another part of me thinks it was right thing to do, clearly I have ways to keep going before I've moved on and she is my ex.. Clearly her workmates will tell her they saw me like a few feet away and that I didn't even say hello, I don't want to seem like jerk, I'm more mature than that, don't want her to think I never want contact ever again, I just don't think its right thing for now.. Sound reasonable.. Or am I just a.. hahaha..

If she had her back to you I would say no harm no foul.. you would have had to go out of your way to speak,maybe by tapping her shoulder and that could be read as an invitation you did not intend.
In the future say hi and do what comes from your gut.. anything else will come off as false and misleading.

yeah if you guys didn't make eye contact or anything you did not reject and invitation to talk to her. And just think, if you had talked to her what awkwardness or regret could have come out of that? Now that the initial shock of randomly seeing your ex has been covered you can prepare yourself in case it happens again and you will be able to go smooth sailing with any conversation!

It happened, let it go, no harm no foul, and you can always say you were busy, or in a hurry, or didn't want to bother her, while she was with her friends. Don't make a big deal out of this.

Don't make a big deal cause it seems to me you probably won't speak to her about it. It really hasn't been that long and No Contact means forever... unless your ex contacts you.

Keep on keeping on and stay happy.