On 9 May, 12:52, "Jo" wrote:

Bee droppings, probably. On the first warm day after winter, they
emerge from the hives in large numbers and unload a whole winter's
worth of bee crap en-masse.
The first warm-enough day would have been some weeks ago. Probably
other insects leave similar yellow deposits as well. I believe they
may also rest on laundry to suck up moisture. Any hives nearby? Is it
under any trees (honey dew, aphid excreta)? Any flowering crops that
would place you under the bee flight-path?