From my understanding, just about anything can be considered an offensive weapon, from a bunch of keys to a brick. It comes down to context and whether you have good reason to have an item on you. A Sgian Dubh worn with the Kilt is considered a good reason but not in heightened security areas.
I normally wear one of my Sgian Dubh bottle openers on non-formal or evening occasions anyway.

Rab
(Sgian Dubh maker)
http://www.rainnea.com/safety-sgian-dubhs