There's a perk in the "gift" category, which adds a couple + tiles to your bag (similar to ones which add -ing or ! tiles). It doesn't come up much, because there are 160+ perks.

Thanks for posting - I haven't seen it at all, either. I guess it's like the wildcard and ing gift tiles, then, though I see those way more often

Haven't seen it either

I'm almost 50 hours in and I haven't seen a single "+" tile! 😭😭😭

Developer didn't test their own game for sht

It's Legendary rarity. If you see it, grab the modifier that makes Rares and Legendaries more likely to appear.

I actually just got it right after I posted. It took 48+ hours to show up but it finally did! I did use the "Rare/Legendary more likely to appear" modifier, even if I tried that a few times before. I even had the "when you add a tile get another' so I was able to complete the achievement for submitting three words using +'s.

Game needs a ton of work