Kim Kardashian Shows Force, Not Jewels for Kendall's Birthday

The only accessory Kim Kardashian's flashing these days is extra hired muscle, which she brought to her sis Kendall Jenner's 21st birthday dinner.

We're told a dressed-down Kim snuck into Catch Wednesday night through a private entrance off the garage, and she was flanked by 3 off-duty cops she hired specifically for the night. Our sources say she dropped about $100/hour for each of them ... plus, each sister's personal bodyguards were also on hand.

There were only about 20 invitees -- including Caitlyn Jenner, Scott Disick and grandma MJ -- in the private room for the party, meaning security guards nearly outnumbered guests. We're told Kim wasn't wearing any jewelry either, as she's been doing since the robbery.

'KUWTK' cameras were inside ... so you'll definitely see Kim on the show -- but for now, she's moving like Big Foot. No one can see her.