Work down this list. Five of the six are timing, and the sixth is the one worth acting on.
1. Payroll landed
Obvious in hindsight, and the most common answer for a business with staff. Two payrolls in one calendar month happens twice a year on a fortnightly cycle and catches people every time.
2. Tax went out
Quarterly or annual, predictable, and rarely planned for as a cash event. If this is a recurring surprise, see tax reserve — moving a percentage of each deposit as it lands removes the whole problem.
3. An annual charge renewed
Insurance, software, licences, vehicle costs. Annual charges are the easiest to forget and the largest single amounts. Recurring payments lists them with their frequency.
4. A large material order
Bought for work not yet invoiced. Cash out now, revenue later, nothing wrong.
5. A big customer paid late
Not a drop in outgoings but a gap in incomings, which feels the same in the balance. Check what is overdue — see overdue invoices.
6. Spending has been drifting up and you noticed at the bottom
This is the one that matters. If none of the above explain it, the fall is not sudden — it is a trend that became visible when the balance got low enough to look at.
Daily transaction velocity and expense trend will show it, and the drop is the symptom rather than the event.
Where to look
Banking health shows inflows against outflows, the top spending categories, and what is due in the next seven days — which is the check worth doing before committing to anything.
Your own numbers
This article explains the general case. For what is actually happening in your business — which customers, which jobs, which period — ask the assistant in the Command Center, which can see your workspace. The wiki cannot: it is public, and it has no access to anyone's data.