Hi, Gary.
Start simple. You didn't say which version of Quicken you are using. I have Q2008 Deluxe, but there should be only minor variations in whatever version you have.
Click Reports | Reports & Graphs Center | Net Worth (under Net Worth & Balances) | Show Report. This should show you - onscreen - the Net Worth Report as of today (10/9/07). Then go back to that same screen and change the date in "Report balance as of:" to yesterday, 10/8/07, and see if the bottom number is unexpectedly different from today. Get the report for each prior day until you see one that takes a big jump. Then print out the two successive days and compare them.
If the unexplained increase happened gradually over the 10-day period, you may have some sleuthing to do. But it more likely happened on a single transaction on a single day, and you should be able to quickly identify that one day. And, by comparing those two days line by line, you should soon see which line - whether asset or liability or something else - took the big jump. By then you know which account and which day, so the rest of the solution should be almost intuitive.
If John's hunch is right - and it probably is - an even easier approach is just to click the Portfolio button. If you don't already use a "today's quotes" "View", you should be able to click Customize View... and create one quickly enough. The important columns to include, of course, are Quote/Price, Shares and Market Value; Day Change or Day Gain/Loss may also help focus on securities that took a jump in value. The default is for Portfolio to show today's numbers; note the total at the bottom of the Market Value column. Then change the "As of:" date to yesterday, then back one day at a time until you see the big change in Market Value and in the day's gain/loss.
Quicken often fails to properly handle stock splits and other such transactions, so watch for those.
Post back with details if you have more questions. And be sure to tell us which version of Quicken.
RC