"Over the pond" doesn't narrow things down much. My experience in England, Ireland, the Netherlands, and Costa Rica has been that the security and customs people were honest, courteous, and professional. Perhaps I've been lucky. But my policy is to hide NOTHING. Something innocent and allowable, in a belt or neck wallet, is apt to at least slow you down. EVERYTHING I carry which is along PSK lines is clearly visable in my carry-on.. in clear plastic bags.

IMHO much of the delay and occasional "attitude" around TSA or customs is simply people trying to "see what they can get by with" and hiding things. For instance: I don't know for certain if my BSA ferro rod fire starter is officially ok. Mine is clearly visable, on the zipper of my carry-on daypack, with a photon led light. Never a problem. But I'll bet there would have been a problem if I'd stuffed it in the toe of a shoe or something.

Incidentally, my large toenail clipper (I did break off the pointed file) has been a better cutting tool than as a nail clipper. Cuts paracord easily. And, it'll produce a spark with my BSA ferro rod. Never brought a second glance.

My signal mirror is with my bic razor. Clear plastic bag. Never a question. My paracord is with my duct tape and a small sewing kit (yep..needles..and carpet thread)... clearly visable. I can't imagine them being questioned: they never have. I carry 2-3 rolls of dental floss. Never been asked to prove that they're for oral hygiene rather than tying stuff up!!

I've carried a water filter pump and carbon filter to Costa Rica. No problem. Can't imagine why I'd even consider bringing one to Europe. But again, I don't know where you're going.

Lighters. Just follow the rules as posted on the link to TSA. A couple of books of matches, and a couple of bic lighters, easily travel without question in my carry-on. Generally nestled in a plastic bag with a few cigars or pipe/tobacco... but not hidden.

Medications/first aid products. If they're OTC here, they're probably OTC there. That's not true coming back to the States. But I don't bring anything I haven't SEEN OTC in both directions.

I do carry some prescription medications such as antibiotics, anti-emetics, and pain medications (which I know how and when to use). I DON'T carry them in their original containers. I know... this isn't the common advice. But I put them in tiny plastic bags (found in the beadwork department of my Hobby Lobby)or I carry the bubble-packed physician samples. I have my pharmacist print an extra "travel" copy of the label...and that goes in the bag. Also, I have a one page listing of these prescription drugs on a Physician's letterhead with his signed note that he has prescribed these medications to me (he had no problems doing this..as it was the truth). Nothing "covert": I'm legally entitled to have them under our DEA regulations, I can prove it, and it has never gone past that.

The only time I've ever had trouble is bringing back a blackthorn walking stick I cut myself on a farm in Ireland. Unvarnished and unprocessed. To my great surprise, the Department of Agriculture checkpoint in Newark was of the opinion that I was possibly bringing in some wood-borer or other pest. Confiscated it. Politely, professionally, cordially. However, they thought the Irish Whiskey was harmless and passed it easily. Little did they know what harm that Powers Gold did to me!