No, it's pretty safe. Munches are held in bars or restaurants during normal hours. Sometimes there's a private room if it's a large group, but usually it's just at one or several big tables. Dress is street casual, not fetish, and conversation is usually discreet. IMO munches are very boring, ;-), but they're pretty emotionally safe. As to physical safety, I suppose you should practice the same precautions as you would at a bar - maybe not let someone mess with your food or drink? But as far as I know that's pretty hypothetical, and not limited to kinky gatherings.

Being a single woman at events like these is only weird the first time. I've done it, at munches, and at play parties too. Dress to be respected - no slut/slave clothes - and behave that way too, and you'll be fine. Honestly, most of the folks at these things are pretty shy and respectful.