What part of the country are you in? Are you near arable land, where crops are grown? Are you under the flight path for an airport? Are these discoloured spots on newer growth of all plants as well?

It does look much like some kind of chemical damage.

Thought it was sun scorch at first but looks too widespread so chemical damage sounds feasible...spraydrift from neighbours ?

I noticed this on my rosemary which is almost beneath next door's ash tree. Several weeks ago the tree was 'pollarded' (in inverted commas because it was a botched up amateurish job), against my advice as it's the wrong time of year. I thought the spotting may have been sap or sawdust. It has almost disappeared now and the leaves are nearly back to their normal green.

Has there been any tree felling near you that could have caused the spotting?

Agree with spray drift.

Yep, spray drift or possibly, aircraft dumping of fuel, which they're not supposed to do over land, but it does sometimes get blown as fine droplets.

Or " Blue Ice " from a faulty waste valve on an airplane but that damage would not only be on your own plant foliage but evenly distributed on you neighbors too.