Does AlarmTag need a clear line of sight for the strobe?

Clip it on the outside if you actually want the strobe seen.

I actually tested this exact thing when I first got mine because I was worried about the same issue. The strobe is bright enough that it still glows through thin fabric, like a t-shirt or a light jacket. I clipped mine to my belt loop under a hoodie once and you could definitely see the flashing through the material. It wasn’t as intense as when it’s fully exposed, but it was noticeable. With something thicker like a winter coat, yeah, that would probably block most of it. So it kind of depends on what you’re wearing. For a keychain setup though, I find that just letting it hang outside my pocket works fine and I don’t really think about it much. The siren is loud enough that the light almost feels like a bonus anyway.

If the strobe is covered, nobody’s seeing it, so it’s basically just running for nothing. That said, the point of the thing is the noise, and 130dB doesn’t care what you’re wearing. I keep mine on a bag strap where it’s actually visible, but I don’t lose sleep over it if a jacket covers it sometimes.

I keep mine clipped to the outside of my bag mostly so the light isn’t hidden. Even a little bit peeking out makes a difference if someone’s looking your way at night.

Sticking it on a belt loop under a shirt is fine if you mostly care about the noise. I had mine on a jacket zip pull for a few months and the strobe caught my eye more than anything else when it went off. That little flash bouncing off a window was actually the first thing I noticed.

I don’t think the strobe being blocked matters much at all. Half the value is just the noise drawing eyes toward you anyway, and once people look over they’ll spot the thing flashing wherever it is. Mine hangs off my bag strap and the light barely shows, still works fine for me.

The strobe really does need to be visible to matter, but that doesn’t mean you have to carry the thing hanging out in the open all day. I got one for my mom last year because she walks early mornings before it’s fully light out, and she keeps it clipped to the strap of her crossbody bag. The light faces out, the bag sits at her hip, and it’s not covered by anything. That was kind of the whole point of picking that spot. She tried clipping it to a jacket zipper first, but the pull tab kept catching on stuff when she reached for things, so the bag strap ended up being the better setup for her.

If your shirt is over it, you’re basically just getting the siren, which is still the loud part. I don’t think there’s a magic answer for carrying it under clothes and still getting the light. Somewhere on the outside of whatever you’re already wearing or carrying is the only way the strobe actually does anything.