The way the law reads, if they have bank information for depositing money, they can use it.  So hypothetically they would use the SS bank account number if it is not on the tax return. 

Whether or not they actually do that or know, who knows.

The law says they look first to the tax returns to determining a payment.  If tax returns are not available, THEN they look to Social Security.  So if a tax return was filed with high income, that would reduce or eliminate the payment, even if they collect Social Security.

If the IRS is 'on the ball', when looking at Social Security for those who have NOT filed a tax return, they SHOULD also look at their other information to see if other income was received by that taxpayer. For example, if a taxpayer did NOT file a tax return and collects Social Security, the IRS would hopefully looks to see if they have any W-2s or whatever else (which means they SHOULD have filed) before they send out the Stimulus payment.  Again, whether or not the do that, we have no idea.  With as much as they are trying to hurry, they may not be doing that.