From the D3O website:
For CE certified D3O® products please follow the washing guidelines issued in the CE booklet user instructions.
For other parts please use the following caution:
Due to the wide ranging use of D3O® and unique stresses that each application imposes on the D3O® components during use, integration etc, it is very difficult to suggest specific washing instructions, appropriate to each and every application. As a general rule we recommend that D3O® protectors are removed carefully from the garment and are wipe-clean only.
For more details please refer to the D3O® Care & Handling Instructions and individual product data sheets.
Also spotted this review saying not removable: http://enduro-mtb.com/en/team-tested-race-face-indy-knee-guards/
Do they have a washing label? If you followed it then warranty.