Sweat from a bike trainer. Outdoor sun usually dries it out, but unless your trainer is in AC, the bike doesn't dry. If the tape is left damp and salty every time it's hard to see the corrosion until it's too late. Happens faster than you'd think. We often put wash cloths/flannels over our bars on the trainer, then set the fan timer to 30m after to dry things out.

Their handlebar is curved like that to stop it from going in too deep..

or got shot.

It's sweat