Tracking pixels are just embedded 1x1 images in HTML emails. They're not hidden; they're just stuffed in the rendered HTML. For example, here's one from B&H photo:
Detecting them automatically is probably tricky, but you can avoid the entire problem by not loading external resources in HTML emails (or, better yet, always load the plaintext version of the email.)
Turn off "auto-load images in emails" option in gmail. If a plain-looking email has that banner at top "click here to load images", there is a pixel-tracker in there.
I don't load images by default. I use Thunderbird, but I know other clients also have this feature. Definitely worth whitelisting domains that you want to allow images from to avoid triggering the recruiter bots!
Agreed, it just is highly unlikely that the LinkedIn resume is out of date for most folks. Asking for a resume without asking if the LinkedIn resume is okay also creates a source of confusion. For example, if they don't ask if the LinkedIn resume will do, are they implying that a LinkedIn resume _won't do_? Do they need me to open up Word and retype my LinkedIn?
* are a part of a large firm
* use multiple fonts, sizes, or any color in their emails
* send an email _and_ an InMail
* text or call you
* jokingly or seriously refer to themselves as a stalker
* automatically substitute in your skills or past company name
* ask for your resume when they can obviously download the LinkedIn pdf
* don’t disclose comp
* don’t disclose the company name
* use tracking pixels or redirect links
* send an automated sequence of follow-up emails (4 follow-ups = bot)
Write them back if they seem like a human! “Not interested at this time, but let’s keep in touch. Thanks for your time” should do.