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Considers?! CONSIDERS?!
That should be standard just as a general public health measure to know what is flowing into the country and not just for covid >

Yeah, it would be a pretty easy way to test for lots of different things at once >

Be ready for them to look for the presence of drugs. >

That would be a good idea to identify trends.  It's not like they can identify the individual responsible. >

Depending on how many segments on your trip they could. >

It is theoretically possible but goddamn there is so much reasonable doubt that would be created that you could fly a planet through it and you would have to test all flights and not just random samples and the drug probably has to be uncommon and you would have to personally drug test anyway in the end. 
But yes, you could in very very unlikely circumstances if you already know who is positive for a rarely used drug find if they were on multiple flights w/drugs in their system.
I'll give a charity upvote for you though because it is theoretically possible still. >

It may just be my cynicism wrt the US justice system that it's possible enough to be used to justify harassment of  someone the police 'know' to be guilty.
But I guess to be fair on that, all they need to do is look at you and decide they smelled some grass to have probable cause. >

Oh, that's actually a pretty good metric. We should also be testing city wastewater >

They do test city wastewater. That's how we have a general ballpark of how widespread certain viruses are.