Try turning the tyre inside out, and using a strong desk light,
closley inspect the tyre inside, by folding it backwards, to open up the tyre inside, and look for any thin bits of steel wire truck tyre reinforcement just sticking into the tyre inside.
If this does not work, try dabbing your finger on the tyre inside, so you could get a pin prick, not cut your finger open!
If you always locate the tyre name next to the valve stem, as the man with the BIG Rubber Mallet, and even bigger Nail Gun says, and you have located and marked the puncture location on the tube, then place the tube with the valve next to the Tyre name, and the puncture causing wire, glass chard, etc, will be located in one of two locations!
I had 20" 406 Marathon Plus tyres on my Scorpion FX trike and they were 2-3 kph+ SLOWER than Kojaks, and they howled when cornering hard and lifting the inside wheel.
The plus tyre as seen below WILL protect you from tacks ets due to the blue urethane protection, but SLOW!!!