If someone accessed your Pi Wallet and transferred Pi out of it, that means they got your passphrase. You may have (unknowingly) entered it into some app or website, or gave access through phishing, malware, a screenshot etc.

Security is only as strong as your secrecy.

Best you can do is create a new Pi wallet and confirm that in Mainnet checklist step 3 + 6, so your NEXT batch of Pi will migrate to that new wallet.