If you wear a thin cotton undershirt as a regular outer shirt, you may see them through it in near-complete darkness. You will also likely see the dark outline of the gun in bright light, or the heavy printing. With normal clothing, no issue.
I tend to doubt it. They aren't that bright. I had a watch with tritium inserts that was pretty bright. It never shone thru a shirt sleeve. Never thought about the rear sight on my gun shining thru a shirt, so I haven't really tested it.
Just tested it in my bathroom in total darkness. Wearing a thin white undershirt and the lights off I press the shirt up against the rear sight, they'll glow through. If it lays like it would if I was normally wearing it, it doesn't at all. So unless you're in 100% darkness with a thin white t-shirt and you intentionally press your shirt against the rear sight, it's not an issue.
SIDE NOTE: It's funny, many people claim (maybe not so much anymore) night sights will give away your position in a low/no light situation. What I have found with sight is if someone can see the glow from your night sights, they can also see you.
Several years ago on GT there was a guy who claimed to be a SEAL who said he could see with his naked eyes night sights a mile away and they would give away your position. I really doubt the guy had ever seen a set of night sights. He sure hadn't seen them at night.
Unless you're in a cave you're never in total darkness.
I have tru glos on my g29 and I am wearing a "wife beater" (thin cotton undershirt) with the lights off the night sights show thru the shirt quite brightly and even show a green glow up a couple of inches on the shirt. The undershirt is nothing I would try to use as a cover shirt. The tru glos are quite a bit brighter than the trijicons I have on several other guns but their locked in the safe and I'm to lazy to pull them out an see how the show! I was actually surprised at how brightly they illuminated the shirt! On a unbuttoned dark grey dress shirt the only show if I push the shirt down on the sights. So my usual cover garments I don't believe anyone would notice the glow.:thumbsup: