If a girl waves at you when you two don't know or talk to each other, is that usually a good sign?

I know everyones different, but what would prompt you to wave at a guy that you aren't really friends with?

This girl at work, we work in to different departments, used to work in the same department before but never talk or anything. I did catch her staring once, and a few months after that I decided to say hi to her.

I work in the yard outside driving trailers around, she drives trucks on the road, and the yards packed with drivers all around so I'll see her briefly every now and then.

Couple weeks ago, as I was backing a trailer, I was trying to pull up to readjust it, but had to wait on some trucks to go past and, she happened to be going past me, so I see her turn her head and stare for about 3 or 4 seconds as she was driving past. She didn't smile, but had this curious anticipation look on her face and she kinda sighed when she turned back around to look at the road.

I didn't know if I did something wrong or not, I didn't smile at her either, I just kinda looked back at her because I was caught off guard and wasn't expecting her.

I felt regret for not at least trying to wave at her to see what would've happened.

A few days ago, I driving in an area to move a trailer from a dock door. I saw her in her truck with her head down waiting for her truck to be loaded, and not to sound creepy i got a little curious so when i past her i quickly looked to see if she was sleeping or not and then i quickly turned away, I'm not sure if she saw that or not.

When I left and had to come back to that same area to grab another trailer and she pulls out where I had to turn in. We had a good bit of distance between us, but I hit the brakes and she stopped I'm guessing to make sure the road way was clear, and when she looked in my direction she waved at me. Not like waving for me to go past her, but like a hi type of wave, and I waved back and afterwards she drove off. Not sure what to make of it, your opinions?

If a girl waves at you when you two don't know or talk to each other, is that usually a good sign?
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