If a bid is past its TTL, is it still valuable to render it? I thought TTL meant "once it's this old, I won't pay for this impression".

if its "past", its gone, non payer etc but i'm thinking and not sure if its even possible, lets say the bid returned has TTL of 600, i'm setting minBidCacheTTL:180, this 180 starts from when bid retunred? but if someone navigates away from page, then back on page 4 mins later the 180TTL has gone but prebid seems to be trying to still "render" it or at least looking for it and throwing this error, could the minBidCacheTTL:180 take into account "off page time", so only counts down on "live" page?

with minBidCacheTTL set to 180, a bid that has TTL of 30 is discarded after 180 seconds; a bid that has TTL of 600 is discarded after 600 seconds (with the caveat that TTL also includes the TTL buffer).

It's plausible that if the window went out of focus in between an auction and the ad render, the ad render may happen after the TTL expired. I think it'd be possible to "pause" the TTL in that case. But I'm not sure it makes sense, I'd expect the buyer to not pay for it.

the "standard"that come back are
banner: 600
native: 300
video: 3600
so my 180 is way below the lowest native 300, so there is a 2 min "buffer" there still for it to be a "live" ad

and/ or i suppose another idea is "split" the minBidCacheTTL into banner, native, video, so can "control" each one

Again, if minBidCacheTTL is set to 180 but the bid expires after 300, it's discarded after 300. minBidCacheTTL is the minimum time bids should be kept in the cache. The issue you're describing can still happen if the window stays out of focus for 300 seconds.

ah ha, i got my min from my expired to my expired + min highly confused, my problems not my min then , doh!