Most solid, non-electrical, objects will not cause interference. These objects include snow, trees, rocks, clothing, skis, etc.  Another common object is a RECCO reflector which is often built into jackets, and this object will not cause interference.

For more info:

Watch this video. See how interference starts as the beacon gets closer than an arm's length away from the phone.

Read the below articles for more information.

“Interference Myth NOT Busted” by John Barkhausen, former GNFAC intern, provides a concise overview of interference and a simply study John conducted while a student at Prescott College.

“Interference Issues Concerning Avalanche Rescue Transceivers” by Manuel Genswein, a Swiss engineer and rescue consultant. This article provides very detailed information on beacon interference as well as a few practical recommendations.