Passive packet sniffing is a simple and low-cost method to collect Wi-Fi data in a specific geographic area. Nevertheless, sniffers may not capture all the Wi-Fi packets due to inherent characteristics of wireless communications, leading to incomplete data traces. We explore the idea of absolute completeness for Wi-Fi passive data capture and validate our findings with indoor and outdoor experimental measurements. We collect Wi-Fi data traces using a controlled source node and multiple sniffers about two meters from the source. We find that individual sniffers have limited capability to capture the traffic and that combining traces from different sniffers improves significantly the data completeness. Deploying redundant, co-located sniffers enhances the completeness but comes at a financial cost. We compare two ways of achieving redundancy: using multiple individual sniffers or using one sniffer with multiple Wi-Fi interfaces. Our experiments show that both ways have similar completeness levels and that we can achieve redundancy with lower cost by using one sniffer with various Wi-Fi interfaces.