I keep my large medical kit in my backback when I'm traveling. It takes up about 1/3 to 1/2 of the pack, and all of my other items sit nicely on top of it. It just happened to be the perfect size to fit horizontally.
I went through Vancouver Airport a few days ago, and as usual, gave myself the pat-down and threw the prohibited items into my carry-on. Backpack went through the x-ray & I was told that something prohibited was inside. I thought "fuck, I forgot to empty the 2-3L water bladder", and told him about the liquid. Turns out they didn't notice the liquid at all; perhaps because of the soft bladder encasement or that it's a smaller daypack that's only giveaway is a small opening for [retracted] hose.
I normally pack the first-aid kit in my carry-on from the start if I'm flying, and forgot about it. It had a cheapo dive knife attached, which was the prohibited item that they found. I was polite, and consented to him to do whatever was needed. Bladder was emptied in the garbage can, & as he was pulling out the kit (and I was thinking "oh fuck...") I noticed his RCR tattoo. We talked about the military (I'm full-time), and he pulled the dive knife off and offered to mail it to me after his shift. Nice guy. Turns out that everything seized from YVR goes to an auction & the proceeds to charity. I gave this one away, & was allowed to carry-on all of my nice medical supplies (various eardrops/eyedrops, epi-pen, ~=25ml glyceryn/anesthetic/antiseptic solutions, and various gels/creams/ointments). These were really my main concern, but everything worked out. I fly all the time, and feel pretty stupid for not doing my usual backpack dump after the pat-down.
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Unrelated, but kind of funny: an infantry guy who works in my building gave his teenager one of his old backpacks. Going through security, it got swabbed and he was taken to a holding room for a couple hours. The forgotten backpack was once used to transport grenades... Apparently he set a new record for explosive residue.
