The IRS has clearly stated that PayPal accounts located outside of the US are considered a foreign bank account for the purposes of FBAR filing. If you have a US-based PayPal account this would normally not need to be reported. Fortunately, the reporting from PayPal makes it pretty easy to find your highest balance for the year. If you go into the “Activity” section and then choose “Statements” (at the top right), you can generate a report for the tax year (dates) in question (use the “Balance Affecting” report) and it will produce a nice summary of the year with the balance after each transaction, where you can locate your highest balance for the year.
PayPal is required to report certain information to the IRS (or the local tax authorities) under the FATCA rules, which means the likelihood of the IRS knowing about a missing account is reasonably high, and the potential penalties are steep! https://www.paypal.com/uk/smarthelp/article/what-does-fatca-require-paypal-to-report-faq3545