A security deposit lands in a bank account and, on paper, it looks exactly like every other dollar in there — available, spendable, part of the balance. It isn't. It's money you're holding on a tenant's behalf, and the day they move out, you owe it back (in full or in part, with documentation either way) on a legally mandated timeline that doesn't care whether it's convenient.
Most self-managing landlords know this in the abstract. Far fewer track it that way in practice — deposits get deposited into the same operating account as rent, blended into the same balance that pays the mortgage and covers repairs, with nothing in the books separating which portion of "cash on hand" is actually spendable and which is already spoken for.
Where this becomes an actual problem
It rarely causes trouble while a tenant stays put. It becomes a problem the week they move out, when you're supposed to return some or all of a deposit within a legally defined window (the exact timeline and rules vary by state, but the obligation is universal) — and if that deposit has effectively already been spent as part of general operating cash, returning it means scrambling, not writing a check from money that was already set aside.
Multiply that across a portfolio with several move-outs happening around the same time, and "scrambling once" becomes a real cash flow squeeze, not a one-off inconvenience.
The fix is visibility, not a new bank account
You don't necessarily need a fully segregated trust account for every deposit (though some states require exactly that) — you need to actually know, at any given moment, how much of your balance is deposits you're holding versus revenue you've actually earned. Most spreadsheet-based bookkeeping doesn't draw that line at all; a dollar is a dollar until it's needed back, and by then it's too late to plan around it.
That's a visibility problem, not a diligence one — the kind of thing that's completely fixable once someone's actually tracking it as its own category instead of letting it blend into the rest of the balance.
If you don't currently know how much of your operating balance is deposits you owe back versus revenue you've earned, that's worth finding out before your next move-out, not during it. Take the free Portfolio Health Check — see where your Turnover Readiness actually stands.