[WP Smush Pro] webp images appearing

Hi. Not sure who I should ask about this, so I"m starting with you. We have offload S3 set to delete images from our server after uploading. We also use wpsmush. Both Seem to have been working fine for months now. Today, for unrelated reasons, I found myself in the Uploads folder/2018... and there are hundreds of "(name).webp" images in each month.
Not blaming it on you at all, but I'd sure love to know where those things are coming from! Supposedly all those images were uploaded to S3 and deleted. Any ideas?