This image is excerpted from a U.S. GAO report:
www.gao.gov/products/GAO-15-73
401(K) PLANS: Greater Protections Needed for Forced Transfers and Inactive Accounts
Note: A plan can only transfer a participant's eligible account to a forced-transfer IRA in the absence of the participant's instruction to do otherwise. An account of less than $1,000 can also be transferred to a forced-transfer IRA, if the plan provides. These balance thresholds pertain to active plans. In the case of terminating plans that must distribute all benefits in order to complete the termination, plans can forcibly transfer missing participants' account balances of any size if the participant, after being notified, does not provide instruction and cannot be located after a search.