Yes, you’re right. Detectable because they have foil.
The dectector’s sensitivity can be adjusted, so it can account for the foil of the packaging. I have a video, using it on Base Set packs, on my iPhone but struggling to upload because of the format.
But a video of someone using one on more modern packs can be found here, when the topic was briefly discussed:
EDIT - Here is the Base Set Video I mentioned