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Here are a couple links that will give you information on a Syn Flood
SYN flood
TCP SYN Flooding Attacks and Remedies - Network Computing
It wouldn't hurt to do a full scan with something like Malwarebytes or MSE
SYN flood can also occur due to an IRC as softwares like this do a check for open ports and proxies. They try and check again and again.
This causes the system to detect SYN flood which is not due to any malware.
What were the running applications when you checked the security log?
BUT, there is no harm in checking.
YBL