Let down or punctured?

Let down push some needles throught the valve cap at various angles ,leave a bit of thread so you can loosely screw it on...

Punctured, no sealant is really permanent (or even very effective when I've had flats). Those stings that look like long turds are ok to get you somewhere for a proper repair though.

Best bet if you suspect a 3rd party is get a camera covering it.