Just reached my office and so far both new tubes haven't lost any air, which means that there's nothing embedded in the tire, and before leaving home, I checked to old front tube which doesn't seem to have lost any air.

FB, thank you for your offer on the o-rings. I'll check and see if the local market have some available, and if not, I'll get in touch with you.

I always use brand name quality tubes (cry once...), but this specific bike still had the original CST tubes installed (the bike is late 2012, early 2013 production, and I did manage to ride the tube over 700 miles).