Occam's razor I agree.

Easy enough to test, just start talking with another person for about 2-5 minutes straight repeating the words purple dog collar over and over in earshot of the device you suspect.

You'll be amazed when within 30 minutes to an hour all the ads on all devices will suddenly show purple dog collars or pet related items.

Really? I info-dump on my partner all the time about all sorts of specific products. She’s never mentioned one randomly pop up in an ad. I can honestly say I’ve performed your test thousands of times.

As a counter-experiment, try thinking of some product and intentionally DON'T search for it or say or type it anywhere. Make sure it's also not something you heard about recently from a friend or in the media. If you then see ads appear for that product you can be sure it's just baader-meinhof.

Talking with another person won't test whether they're correlating via IP or location.